Update building-a-python-destination.md

[lukehsiao]

What

Describe what the change is solving
It helps to add screenshots if it affects the frontend.

If a user is following the documentation and used append_dedupe, they get the error:

{
  "type": "LOG",
  "log": {
    "level": "FATAL",
    "message": "1 validation error for ConnectorSpecification\nsupported_destination_sync_modes -> 2\n  value is not a valid enumeration member; permitted: 'append', 'overwrite', 'append_dedup' (type=type_error.enum; enum_values=[<DestinationSyncMode.append: 'append'>, <DestinationSyncMode.overwrite: 'overwrite'>, <DestinationSyncMode.append_dedup: 'append_dedup'>])..."
  }
}

How

Change the docs to show the right value :).

[CLAassistant]

All committers have signed the CLA.

[marcosmarxm]

Hello 👋, first thank you for this amazing contribution.

We really appreciate the effort you've made to improve the project.
We ask you patience for the code review. Last month our team was focused on Hacktoberfest event and that probably left some PR without the proper feedback. And this week, due to the Thanksgiving US Holiday, most our team is out of office with their families. Another important piece of information why code won't be merge this week is: as a safety measure the core team has decided to freeze merging code to main branch to keep the release stable. Next week we'll return to you with the proper code review and update the status of your contribution.

If you have any questions feel free to send me a message in Slack!
Thanks!